Sulfonation of aromatic hydrocarbons is a commercially important process for the production of cation-exchange materials, anionic detergents, and lubricating oil additives. An example of such products is sodium dodecylbenzenesulfonate, a principal ingredient of laundry detergent formulations.
Sulfonation of the dodecylbenzene is done commercially either by heating the dodecylbenzene with 20% oleum or contacting it with vaporized sulfur trioxide.
The oleum process generally involves mixing the hydrocarbon with 20% oleum at 35.degree.-50.degree. C. for 2 hours. After addition of a small amount of water to separate layers, neutralization of the organic layer with sodium hydroxide yields a product of 88-90% sodium dodecylbenzenesulfonate and 10-12% sodium sulfonate. Separation and disposal of the sodium sulfate creates problems.
The processes utilizing sulfur trioxide involve lower-cost reagents and yield a product which contains less sodium sulfate. However, the reaction is very exothermic and somewhat more difficult to control. Liquid sulfur trioxide cannot be added directly to the aromatic hydrocarbon without extensive charring and dealkylation of the alkylaromatic. Normally, sulfur trioxide is used at a concentration of 5% in an inert gas such as dry air or dry nitrogen. Efficient stirring of the viscous mass is required to promote gas-liquid contact throughout the reaction mass. This is particularly difficult at the end of a batch reaction because of the extremely viscous product. Although reaction occurs readily at low temperature, a temperature of about 50.degree. C. is used to reduce viscosity. Neutralization of the reaction mass yields a product which is substantially sodium dodecylbenzenesulfonate in aqueous solution.
Membranes of various types have been used to carry out certain separations of aqueous solutions. An example of this type of separation is the desalination of sea water by forcing it under pressure through a membrane.
It has now been found that certain membranes may be used in non-aqueous environments and under conditions which permit the selective contacting of the reagents to easily carry out reactions which hithertofore were difficult to control. This discovery provides a simple and efficient process for the sulfonation of organic compounds which avoids the problems associated with the known processes while providing a sulfonated product which is relatively free of undesirable by-products.
In my prior patent (U.S. Pat. No. 3,976,704) was described and claimed a process of nitration of organic compounds wherein the nitrating agent, usually aqueous nitric acid, and organic compound are substantially separated by a polymeric membrane preferably comprising sulfonic acid groups; when the preferred perfluorosulfonic acid polymer is used as the membrane, the sulfonic acid groups in the membrane supply the necessary catalytic acid and the organic aromatic compounds can be nitrated without the incorporation of strong sulfuric acid in the nitrating agent. Such nitration reaction inherently is in an aqueous environment not only from the water in the usual aqueous nitric acid as the nitrating agent but also from the water produced in the nitration reaction. As is well known, a catalyst is necessary in nitration to create the nitronium ion as the attacking agent in the reaction; without the catalyst the reaction does not take place. On the other hand the present sulfonation process is in a substantially anhydrous environment and the preferred sulfonating agent, sulfur trioxide, adds without generating water. As indicated above the direct contact of sulfur trioxide to an aromatic compound gives a strongly exothermic reaction and excessive formation of by-products and char; for example, direct reaction of benzene with sulfur trioxide was heretofore impractical because of excessive by-product formation; and heretofore direct contact of detergent alkylate with sulfur trioxide caused much prohibitive dealkylation. Many other differences between my patented nitration process and the present sulfonation process will be apparent to one skilled in the art.